Washington: Two different compounds found in marijuana appear to have opposite effects on certain regions of the brain, according to a new study led by an Indian-origin scientist.

While one chemical, known as tetrahydrocannabinol (THC), increases the brain processes that can lead to symptoms of psychosis, another compound, called cannabidiol, negates such symptoms, found the study led by Dr Sagnik Bhattacharyya, a psychopharmacologist at King's College in London.

Moreover, the findings, published in the journal Archives of General Psychiatry, are the first to use images of the brain to demonstrate that the reason symptoms of psychosis arise in marijuana users may be because THC interferes with the brain's ability to distinguish between stimuli that are important, and those that aren't, the researchers said.

Past research has found that THC can induce symptoms of psychosis in healthy people and worsen psychotic symptoms in people already experiencing them. Long-term cannabis use is also associated with an increased risk of schizophrenia.

In terms of brain processing, researchers said, psychotic symptoms have been linked to what researchers call abnormal "salience attribution," meaning that the brain has difficulty telling the difference between stimuli that are important, and those that aren't, as reported.

For the study, the team included 15 health men who had occasionally used marijuana in the past. They used functional magnetic resonance imaging (fMRI) scans to observe the men's brains after they took pills containing THC, cannabidiol or a placebo.

While inside the FMRI scanner, the volunteers performed a computer task designed to measure their abilities to respond to a certain stimulus that was different from others. The images showed changes in the brain areas that are believed to be linked to symptoms of psychosis, the researchers said.

The results showed that THC "significantly increased the severity of psychotic symptoms compared with placebo", while there was no difference in psychotic symptoms seen between taking cannabidiol and a placebo, they said.

The study, according to the researchers, showed that men taking THC had increased activity in the brain region called the prefrontal cortex, but lower activity in the region called the striatum.

It is possible that these changes happened because THC alters the brain's levels of the neurotransmitter dopamine, according to the study.

"Altered prefrontal-striatal interactions are thought to be critical in the pathophysiologic characteristics of psychosis," the authors wrote.

This is consistent with evidence that striatal and lateral prefrontal function are altered during salience processing in patients with psychosis, individuals at ultrahigh risk of psychosis, and persons in a drug-induced psychotic state

On the other hand, the effects that cannabidiol had on the brain suggested it has the opposite effect on psychotic symptoms. In line with other studies, the new results suggest the compound may have potential as an antipsychotic, the authors wrote.